1. Field of the Invention
This invention pertains to optical hybrids generally, and specifically to hermetic packaging techniques which allow for precision alignment of optical interfaces without the usual associated elevated expense.
2. Description of the Related Art
Optical devices may either transmit, conduct or receive light energy. The light energy may, for the purposes of this disclosure, be concentrated in the band visible to the human eye, or in regions of both shorter and longer wavelengths.
In present day commerce, the sciences of optics and electronics are often synergistically combined to produce products which have performance characteristics which are uniquely benefited. This combination offers numerous advantages. These products may offer higher operational speeds and increased bandwidths, exemplified by the optical fiber telecommunications industry. Optical links are also much more immune to signal interference caused by stray electro-magnetic waves than are electrical links. Additional advantages which are too numerous to mention may be found in any text on opto-electronics. As a result of the many advantages, there is a desire to incorporate optical devices and systems into traditional electronic packages.
Best performance of individual optical devices in an optical system is generally obtained where a maximum amount of light energy is coupled between transmitter and receiver. A good strong signal generated by a transmitter and efficiently coupled to a receiver will allow for a high signal-to-noise ratio.
This light energy must be coupled in such a way as to not adversely affect the phase relationship of the various waves within the light, in those systems which modulate the light energy at significant frequencies. Additionally, large distances between transmitters, conductors and receivers may provide a source for undesirable reflections or diffusion.
Many designs have been produced which attempt to incorporate the above and other considerations into an optical interface hybrid. Illustrative of the designs is U.S. Pat. No. 4,836,635 to De Amorim. In this design an optical device is mounted inside a package having an optically transparent window. In order to focus the light and reduce the possibility of reflection, a concentrating lens is placed between the optical component and the transparent window. Alignment of the various components is critical and as a result, the package is necessarily complicated and difficult to manufacture. Additional expense is also necessarily incurred by the inclusion of the concentrating lens.
Another common type of opto-electronic package is illustrated in U.S. Pat. No. 4,897,711 to Blonder et al. A package having an alignment hole for optical cable is provided, and a reflector is interposed between the optical cable and the receiving device to provide redirection of the light from the cable to the receiver in a reliable way. However, such a package suffers from the limitation of being very difficult to reliably hermetically seal during production. Where the parts are successfully sealed, delivery to the customer becomes an entire engineering feat in and of itself, because of the "pig-tail" created by the protruding optic cable. In spite of the problems created by the package design, the customer must still incorporate some type of optical coupling to the optical cable. While this type of package design is satisfactory for those applications which do not require hermeticity, the design is inadequate for hermetic packages. U.S. Pat. No. 4,846,930 to Stanley discloses a similar arrangement for a laser diode.
Several packaging schemes provide for coupling from an optical device through an opening or optical window in the package. Illustrative of this concept are U.S. Pat. Nos. 4,929,965 to Fuse and 4,893,901 to Taumberger.
Fuse discloses a hermetic package in which an optical device is mounted and aligned with an optical window. Yet no alignment mechanism is provided to assist in the manufacture of the device. In production, these devices are usually assembled with the provision of a test signal or very expensive, tightly controlled placement tolerance equipment. Additionally, the device is directly attached to the package structure with the adhesive in close proximity to the window, resulting in potential interference between the adhesive component and the optical window. Such susceptibility to adhesive bleed will adversely affect the yield of components which contain a large material investment.
Overcoming many of the adversities of other designs, Taumberger et al incorporate an optical window on a side of the package which is opposite to the optical device. Alignment is less critical, since a variation in the incoming incident angle of the light energy will vary the location on the substrate where the light will be most concentrated. In other words, the package may be tilted slightly relative to the parts of the optical system which are exterior to the package in order to align a relatively columnated beam of light between the optical device within the package, the optical window, and the optical device(s) exterior to the package. However, the Taumberger design is not without material limitations. The additional spacing created between the window and the optical device forces alignment with the outside to be very critical. There are three points which must be brought into alignment--the window, the internal optical component and the external optical component. Additionally, either the package or the external optical device must be adjustable. Adjustment will be generally required after assembly, necessitating extra cost and potentially more elaborate mounting for the package or the external device.